The reported data indicate that oleic acid (OA) decreases cholesterol absorption. To explore the underlying mechanisms, the effects of OA on the expression of cholesterol transport-related proteins (NPC1L1, ABCG5/8, ACAT2, MTP) and the unfolded protein response (UPR) pathway were studied in CaCo-2 enterocytes by incubating CaCo-2 cells with taurocholate micelles or taurocholate micelles containing different concentrations of OA (0.25-1.0 mM). We show that OA effectively induces XBP1 mRNA splicing, a key component of the UPR signaling, and the expression of BiP and mature ATF6 proteins in a concentration-dependent manner, leading to the induction of endoplasmic reticulum (ER) stress and activation of the UPR. Interestingly, OA decreases NPC1L1 expression in a dose-dependent manner while it has no effects on ABCG5 and MTP mRNA level or SREBP-2, ABCG8, and ACAT2 protein level. In CaCo-2 cells treated with 1.0 mM OA, both the NPC1L1 mRNA level and the NPC1L1 protein expression in brush-border membrane fractions were decreased by 39% and 37%, respectively (P < 0.01). A dose of 1 mM dithiothreitol (DTT), a positive control for ER stress induction, also decreases NPC1L1 mRNA and protein expression by 27% and 23%, respectively (P < 0.05). Furthermore, 4-phenyl-butyric acid, an UPR inhibitor, blocks OA- and DTT-induced reduction on NPC1L1 mRNA and protein levels. The results suggest that OA down-regulates NPC1L1 mRNA and protein expression via the induction of the UPR, which may play an important role in reducing intestinal cholesterol absorption.